For alloys which are subject to weight segregation due to incorporation of additive elements having a great difference in specific gravity or alloys containing elements which are segregation during solidification, it has been desired to effectively prevent occurrence of segregation during casting in order to improve product quality. In particular, Mg alloys and other light alloys are subject to the aforementioned segregation.
Referring to segregation of alloying elements that occurs during casting, a method can be proposed which includes lowering a casting temperature in order to reduce a time required until solidification, eliminate crystal growth or prevent phases having a high specific gravity or elements having a great atomic weight from settling. However, in actual casting, fluidity of molten metal in a thick shape or complicated shape must be taken into account. Thus it is difficult to substantially lower the casting temperature. Further, in a simple shape such as ingot, the casting temperature can be lowered. However, such a material is normally thick and thus needs a long solidification time even if the casting temperature is somewhat lowered. Thus, segregation occurs inevitably.
Besides this method, a method has been proposed which comprises eliminating segregation or finely dividing texture during casting to indirectly eliminate segregation (e.g., Patent Documents 1, 2).
The method disclosed in Patent Document 1 is a casting method which comprises effecting solidification in a horizontal direction in a directional solidification furnace having a chilling plate and a heating furnace while being slowly rotated on a horizontal axis. Thereby, casting defectives such as shrinkage cavity and segregation are eliminated while providing a sufficient temperature gradient.
Further, the method disclosed in Patent Document 2 is a casting method which comprises repeating mincingly rotation/reversed rotation or rotation in one direction and suspension of rotation of a casting mold placed on a chilling plate (water-cooling plate) so that a horizontal vibration is given to the casting mold in order to produce a fine regular system structure.    Patent Document 1: JP-A-2000-343204    Patent Document 2: JP-A-2002-331354